Historical Geology, Geological Time and Palaeoenvironmental Reconstruction
This domain tests whether you can order events in time and reconstruct past environments from rocks and fossils. Expect questions on stratigraphic principles, the geological time scale, absolute dating methods and palaeoclimate proxies.
Principles of stratigraphy
- Superposition: in an undisturbed sequence, the lowest bed is the oldest.
- Original horizontality and lateral continuity (Steno).
- Cross-cutting relationships: a fault or intrusion is younger than what it cuts.
- Inclusions: a clast inside a rock is older than the rock containing it.
- Faunal succession: fossil assemblages change in a fixed order and allow correlation.
- Unconformities: angular (tilted beds truncated below), nonconformity (sediments over igneous/metamorphic basement), disconformity/paraconformity (parallel but with a gap).
The time scale
Hadean, Archean, Proterozoic, Phanerozoic. Phanerozoic eras: Palaeozoic (Cambrian to Permian), Mesozoic (Triassic, Jurassic, Cretaceous), Cenozoic (Palaeogene, Neogene, Quaternary with Pleistocene and Holocene). Learn the boundaries you are likely to be asked: end-Permian (~252 Ma), end-Cretaceous (~66 Ma), Pleistocene/Holocene (11.7 ka).
Absolute dating
- Radiometric: U-Pb in zircons (old rocks, high precision), Rb-Sr, K-Ar and Ar-Ar, Sm-Nd.
- Carbon-14 works only on organic remains younger than about 50,000 years.
- Other tools: varves, tree rings (dendrochronology), magnetostratigraphy, ice-core layers, tephrochronology.
Palaeoenvironmental proxies
Lithology and sedimentary structures (ripples, cross-bedding, evaporites), fossils and biofacies (reef corals, planktonic foraminifera, pollen), stable isotopes (δ18O for temperature and ice volume, δ13C for the carbon cycle), and sequence stratigraphy for sea-level change.
Worked example
A section shows folded marls with Miocene planktonic foraminifera, truncated by an irregular erosion surface, then horizontal conglomerates containing pebbles of the marls. Reading: the marls are Miocene marine deposits; the erosion surface is an angular unconformity, so deformation and uplift occurred after the Miocene; the conglomerate is younger still, and its marl clasts confirm the underlying unit was already lithified and exposed. Order of events: deposition → folding → erosion → conglomerate deposition.
Traps that cost marks
- Using C-14 on dinosaur bone or on limestone.
- Confusing era, period and epoch.
- Claiming the conglomerate is older because it lies at the base of a cliff.
- Ignoring that an unconformity represents missing time, not a single surface.